Untitled

Value Difference (Aggregated) = 
SUMX(
    SUMMARIZE(
        'Combined Table',
        [Unified Key],
        "AggValue", MAXX(FILTER('Combined Table', [Source] = "Aggregated"), [Value]),
        "QMMeteredValue", MAXX(FILTER('Combined Table', [Source] = "QM-Metered"), [Value])
    ),
    COALESCE([AggValue], 0) - COALESCE([QMMeteredValue], 0)
)